In recent years, a system using a transparent conductive oxide as a main component such as, for example, zinc oxide (ZnO) has been actively developed as a material for the active layer of a field effect transistor (Japanese Patent Application Laid-Open No. 2002-76356).
Zinc oxide (ZnO) is allowed to be thin-filmed at a comparatively low temperature. The development of a flexible transistor using a flexible substrate such as polymeric substrate has been attempted.
For example, Japanese Journal of Applied Physics, Volume 42, No. 4A (2003) pp. L347 to L349 discloses that a CaHfOx buffer layer is used to obtain a good TFT characteristic at a substrate temperature of 150° C. However, it does not mention whether deposition is made at a room temperature to obtain a good TFT characteristic.
Since ZnO tends to create the vacancy of oxygen and is sensitive to the atmosphere (or, there occur changes in its characteristic due to the influence of the atmosphere), a protective layer is provided to separate an active layer from the atmosphere according to, for example, U.S. Patent Application Publication No. 2006/244107A1.
The above patent publication describes that the protective layer may be provided with a region so that electric charges (also called movable electric charges) in the active layer move are separated from the atmosphere so as not to be influenced by the atmosphere.
For this reason, the protective layer is formed to suppress the influence of the atmosphere on the active layer after a semiconductor device has been produced, but not to suppress the absorption of moisture into the active layer during deposition.
The inventors made a study of a practical application of an insulated gate type transistor using an oxide semiconductor in its active layer by means of the sputtering method, and the inventors' study has revealed that even insulated gate type transistors produced by the same apparatus and under the same conditions differ in mobility and on-off ratio depending on the time when they were produced.
The present invention has for its object to provide an insulated gate type transistor which is high in mobility and good in on-off ratio, and a display device using the same.